Jelly-roll type electrode assembly and secondary battery including the same

ABSTRACT

An electrode assembly including a first electrode strip having a first electrode collector coated with at least a first electrode active material, an exposed portion of the first electrode collector attached with a first electrode tab; a second electrode strip having a second electrode collector coated with at least a second electrode active material and is rolled together with the first electrode strip, an exposed portion of the second electrode collector is attached with a second electrode tab; and at least one inter-electrode strip separator is positioned between the first and second electrode strips, wherein at least one sheet of protective separator, which is extended from the inter-electrode strip separator, is further positioned on a side of the first electrode strip attached with the first electrode tab.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.2003-57277 filed on Aug. 19, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a jelly-roll type electrode assemblyand a secondary battery including the same, and particularly to, ajelly-roll type electrode assembly in which short-circuiting betweenelectrodes can be prevented, and a secondary battery including the same

2. Description of the Related Art

In general, secondary batteries, which are distinguished from primarybatteries by their ability to be repeatedly charged and discharged, havebeen widely used in cellular phones, notebook computers, camcorders, andother portable electronic devices. Lithium secondary batteries having anoperating voltage of 3.6V or greater, which is three times higher thannickel-cadmium (Ni—Cd) batteries, have become popular as a power sourcefor various kinds of electronic equipment and nickel-hydrogen batteriesand, are frequently used because of their high energy density per unitof weight.

Such lithium secondary batteries mostly use a lithium oxide as apositive active material and a carbonaceous material as a negativeactive material. Lithium secondary batteries can be classified intoliquid electrolyte batteries, also known as lithium ion batteries, andpolymer electrolyte batteries, also known as lithium polymer batteries,according to the type of electrolyte used. Lithium secondary batteriesare manufactured in various shapes, typically, in cylindrical,rectangular, or pouch forms.

A second battery uses a jelly-roll type electrode assembly manufacturedby applying an active material onto a substrate, drying the substrate,pressing it using a roller, and severing it into a positive electrodestrip and a negative electrode strip, interposing a separator betweenthe positive and negative electrode strips to make a multi-layerstructure, and rolling the multi-layer structure in a jelly-roll form. Acylindrical battery is made by placing such an electrode assembly into acylindrical can, injecting an electrolyte into the cylindrical can, andsealing the cylindrical can. A rectangular battery is made by applying apressure to such an electrode assembly so that it has plane surfaces,and then, placing the electrode assembly into a rectangular can.

In such an electrode assembly, a positive electrode tab and a negativeelectrode tap are drawn out from the positive electrode strip and thenegative electrode strip, respectively. In particular, one of thepositive and negative electrode tabs is drawn out upward from itsrelated electrode strip and electrically connected to a cap assembly forsealing the can. The other electrode strip is drawn out downward fromits related electrode strip and electrically connected to a projectionformed at the bottom of the can. In the case of the rectangularsecondary battery, both the positive and negative electrode tabs may bedrawn out upward from their electrode strips.

As described above, a conventional cylindrical battery includes anelectrode assembly made by interposing only a separator between a majorelectrode strip and an auxiliary electrode strip to make a multi-layerstructure and rolling the multi-layer structure. The separator is,however, prone to be broken at portions of the positive and negativeelectrode strips welded together with electrode tabs, thus causingelectrical short-circuiting. An electrode tab enables movement ofelectric charges from a battery to the outside, and therefore, whenelectric charges are concentrated on the electrode tab, the electrodetab is heated. As a result, an electrode strip attached with the heatedelectrode tab is also heated, thus resulting in breaking of theseparator. Then, although the electrode strip attached with theelectrode tab is separated via the separator from the other electrodestrip of different polarity, the breaking of the separator causeselectrical short-circuiting between the heated electrode strip and theother electrode strip.

Also, when a defect, such as a burr, forms at an end of an electrode tabduring manufacture of the electrode tab, the burr damages the separator,especially a thin separator, thus causing an electrical short-circuitingvia a damaged portion of the separator.

Japanese Patent Publication No. 11-273660 has suggested a batteryelectrode assembly in which a portion of an electrode strip weldedtogether with an electrode tab is coated with a polymer material,thereby preventing a separator from being damaged due to the formationof a burr or other defect of the electrode tab. However, in this case,since a conventional battery manufacturing process further requires apolymer coating process, the manufacture process is more complicated andmanufacturing costs are increased.

Japanese Patent Publication No. 4-109551 discloses a jelly-roll typebattery electrode assembly in which an electrode tab is bent inward morethan the rolled battery electrode assembly and a bent portion of theelectrode tab is located at a center portion of an inner diameter of therolled electrode assembly. However, it is very difficult to position theelectrode tab at the center of the rolled electrode assembly. Even ifthe electrode is positioned at the center, it is difficult to insert awelding rod into the electrode assembly due to the presence of theelectrode tab during a subsequent process of welding an electrode tabtogether with a base plane of a can.

U.S. Pat. No. 5,508,122 discloses a secondary battery with a spiralelectrode unit. In the spiral electrode unit, same-polarity regions ofan electrode strip attached with a lead toward a center portion of thespiral electrode unit are positioned via a separator on both sides ofexposed regions of electrode core material, and a lead attached to theother electrode strip is positioned at the outmost winding of the spiralelectrode unit. Accordingly, even if the separator is damaged,electrical short-circuiting can be prevented.

However, in the spiral electrode unit, the exposed regions must be longenough to position the same-polarity regions of the electrode strip viaa separator on both sides of the exposed regions, and a starting pointof rolling the other electrode strip is later than that of rolling theelectrode strip. Thus, the secondary battery includes many portions ofelectrode strips unnecessary for battery reaction, thereby increasingmanufacturing costs. If the secondary battery is rectangular shaped,that is, when a length of the battery in circumference is long, thisproblem becomes more serious.

SUMMARY OF THE INVENTION

The invention provides a jelly-roll type electrode assembly designed toreduce electrical short-circuiting at a portion of an electrode stripwelded together with an electrode tab, and a secondary battery includingthe same.

The invention also provides a jelly-roll type electrode assembly inwhich electrical short-circuiting can be prevented even if an electrodetab has a surface defect such as a burr, and a secondary batteryincluding the same.

The invention also provides a jelly-roll type electrode assemblyfabricated using conventional equipment but electrical short-circuitingbetween electrode strips can be prevented, and a secondary batteryincluding the same.

According to an aspect of the invention, there is provided an electrodeassembly comprising a first electrode strip including a first electrodecollector coated with at least a first electrode active material, anexposed portion of the first electrode collector attached with a firstelectrode tab; a second electrode strip which includes a secondelectrode collector coated with at least a second electrode activematerial and is rolled together with the first electrode strip, anexposed portion of the second electrode collector attached with a secondelectrode tab; and at least one inter-electrode strip separator issandwiched between the first and second electrode strips. Here, at leasta sheet of protective separator, which is extended from theinter-electrode strip separator, is further positioned on a side of thefirst electrode strip attached with the first electrode tab.

The first electrode tab may be formed at a center of the rolledelectrode assembly. At least one of the inter-electrode strip separatorand the protective separator is positioned on an opposite side of theside of the first electrode strip attached with the first electrode tab.

The at least one protective separator may be an end of theinter-electrode strip separator. The at least one protective separator,which is the end of the inter-electrode strip separator, is bent morethan one time, for example twice, and sandwiched between the first andsecond electrode strips.

The first electrode tab may be positioned at a side of the firstelectrode collector a center of the electrode assembly.

The at least one protective separator may be extended from theinter-electrode strip separator while being rolled in the oppositedirection in which the electrode assembly is rolled.

The electrode assembly is a rolled structure in which a sheet of theinter-electrode strip separator, the first electrode strip, a sheet ofthe inter-electrode strip separator, and the second electrode strip aresequentially multi-layer, wherein the at least one protective separatoris extended from at least one of the two sheets of inter-electrode stripseparators.

Starting points of applying the first and second active materials may bethe same.

The second electrode strip may be positioned via the protectiveseparator on a side of the first electrode strip attached with the firstelectrode tab.

The at least one protective separator may be extended to contact theopposite side of a side of the inter-electrode strip separator, whichresides between the side of the first electrode strip attached with thefirst electrode tab and the second electrode strip, toward the center ofthe electrode assembly, the at least one protective separator positionedon at least the side of the first electrode strip attached with thefirst electrode tab.

According to another aspect of the invention, there is provided anelectrode assembly comprising a first electrode strip including a firstelectrode collector covered with at least a first electrode activematerial, an exposed portion of the first electrode collector attachedwith a first electrode tab; and a second electrode strip including asecond electrode collector covered with at least a second electrodeactive material, an exposed portion of the second electrode collectorattached with a second electrode tab. The respective first and secondelectrode strips are sandwiched between two sheets of separators andpositioned on one of the two sheets of separators so as to make amulti-layer structure, the multi-layer structure is inserted into amandrel and rolled using the mandrel, and at least one of the two sheetsof separators is inserted into the mandrel and its portion is drawn outfrom the mandrel by at least half a length the mandrel in conference,and is rolled using the mandrel.

The drawn portion may be extended to the side of the first electrodestrip attached with the first electrode tab.

The second electrode strip may be positioned via the drawn portion ofthe separator and the other separator on the side of the first electrodestrip attached with the first electrode tab.

Only the separators may be present on the opposite side of the side ofthe first electrode strip attached with the first electrode tab.

The length of the drawn portion of the separator is preferably longerthan a sum of half the length of the mandrel in circumference and adistance between the mandrel and the side of the first electrode stripattached with the first electrode tab.

The drawn portion of the separator may be rolled at least half a roundof the mandrel, and then the first electrode strip is rolled.

The first electrode tab may be positioned at a side of the firstelectrode collector toward a center portion of the electrode assembly.

The drawn portion of the separator may be extended while being wound inthe opposite direction in which the electrode assembly is rolled.

According to another aspect of the invention, there is provided anelectrode assembly comprising a first electrode strip including a firstelectrode collector coated with at least a first electrode activematerial, an exposed portion of the first electrode collector attachedwith a first electrode tab; a second electrode strip which includes asecond electrode collector coated with at least a second electrodeactive material and is rolled together with the first electrode strip,an exposed portion of the second electrode collector attached with asecond electrode tab; and at least an inter-electrode strip separatorpositioned between the first and second electrode strips. At least asheet of protective separator, which is formed to be united with the atleast one inter-electrode strip separator, and the exposed portion ofthe first electrode collector are further positioned at a side of thefirst electrode strip attached with the first electrode tab.

The first electrode tab may be positioned at a center of the electrodeassembly. At least one of the at least one inter-electrode stripseparator and the at least one protective separator may be positioned atthe opposite side of the side of the first electrode strip attached withthe first electrode tab.

The at least one protective separator may be an end of the at least oneinter-electrode strip separator. The at least one protective separator,which is the end of the at least one inter-electrode strip separator,may be bent twice and sandwiched between the first and second electrodestrips.

The exposed portion of the first electrode collector may be extended tothe side of the first electrode strip attached with the first electrodetab and bent twice and interposed between the first and second electrodestrips.

The at least one protective separator may be extended from theinter-electrode strip separator while being wound in the oppositedirection in which the electrode assembly is rolled.

The exposed portion of the first electrode collector may be extended tothe side of the first electrode strip attached with the first electrodetab while the exposed portion being wound in the opposite direction inwhich the electrode assembly is rolled.

The electrode assembly may be a rolled structure in which a sheet of theinter-electrode strip separator, the first electrode strip, a sheet ofseparator, and the second electrode strip are sequentially positioned,wherein the protective separator is extended from at least one of thetwo sheets of separators.

Starting points of applying the first and second electrode activematerial may be the same.

The second electrode strip may be positioned via the protectiveseparator and the exposed portion of the first electrode collector onthe side of the first electrode strip attached with the first electrodetab.

According to another aspect of the invention, there is provided anelectrode assembly comprising a first electrode strip including a firstelectrode collector coated with at least a first electrode activematerial, an exposed portion of the first electrode collector attachedwith a first electrode tab; and a second electrode strip including asecond electrode collector coated with at least a second electrodeactive material, an exposed portion of the second electrode collectorattached with a second electrode tab. The respective first and secondelectrode strips are sandwiched between two sheets of separators orpositioned on one of the two sheets of separators so as to make amulti-layer structure, the multi-layer structure is inserted into amandrel and rolled using the mandrel, and at least one of the two sheetsof separators and the exposed portion of the first electrode collectorare inserted into the mandrel and their portions are drawn out from themandrel by a length equal to at least half a circumference of themandrel, and are rolled.

The drawn portions may be extended to a side of the first electrodestrip attached with the first electrode tab.

The second electrode strip may be positioned on the side of the firstelectrode strip attached with the first electrode tab, via the drawnportion of the separator and at least a separator between the first andsecond electrode strips.

The drawn portion of the extended portion of the first electrodecollector may be extended to the side of the first electrode stripattached with the first electrode tab, wherein the second electrodestrip is positioned via the drawn portion of the separator on a side ofthe drawn portion of the extended portion of the first electrodecollector.

A length of the drawn portion of the exposed portion of the firstelectrode collector may be longer than a sum of half a length of themandrel in circumference and a distance between the mandrel and the sideof the first electrode strip attached with the first electrode tab, anda length of the drawn portion of the separator may be longer than thatof the drawn portion of the exposed portion of the first electrodecollector.

The drawn portion of the exposed portion of the first electrodecollector may be wound half a round of the mandrel, and, then, the firstelectrode strip is rolled.

The first electrode tab may be positioned on a side of the firstelectrode collector toward a center portion of the electrode assembly.

The drawn portions of the separator and the exposed portion of the firstelectrode collector may be drawn out from the mandrel while the drawnportions are wound in the opposite direction in which the electrodeassembly is rolled.

The separators may include a material selected from a group includingpolyethylene, polypropylene, and a copolymer of polyethylene andpolypropylene, have a multi-layered structure, or include apolyolefin-based polymer with a molecular weight of 350 thousand orgreater.

An active material may not be applied onto a portion of the first orsecond electrode strip which resides on the other adjacent second orfirst electrode strip of different polarity via an active material ofthe other electrode strip and the separators.

The first active material may include a lithium-based oxide and thesecond electrode active material comprises a carbon-based material. Thefirst electrode collector may be formed of aluminum or an aluminum alloyand the second electrode collector may be formed of copper or a copperalloy.

According to another aspect of the invention, there is provided asecondary battery including an electrode assembly placed in acylindrical or rectangular can or a pouch case.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of the embodiments taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a cross-sectional view of a cylindrical secondary batteryincluding an electrode assembly according to an embodiment of theinvention;

FIG. 2 is a cross-sectional view of a center portion of an electrodeassembly according to an embodiment of the invention;

FIG. 3 illustrates a method of rolling the electrode assembly of FIG. 2according to an embodiment of the invention;

FIG. 4 is a cross-sectional view of the center portion of the electrodeassembly of FIG. 1;

FIG. 5 is a cross-sectional view of the center portion of the electrodeassembly of FIG. 2;

FIG. 6 is a cross-sectional view of a center portion of an electrodeassembly according to another embodiment of the invention;

FIG. 7 illustrates a method of rolling of the electrode assembly of FIG.6 according to an embodiment of the invention;

FIG. 8 is a cross-sectional view of the center portion of the electrodeassembly of FIG. 6;

FIG. 9 is another cross-sectional view of the center portion of theelectrode assembly of FIG. 6; and

FIG. 10 is another cross-sectional view of the center portion of theelectrode assembly of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below to explain the presentinvention by referring to the figures.

The invention is described with respect to a cylindrical secondarybattery with a jelly-roll type electrode assembly; however, theinvention is not limited to this disclosure. For example, if a batterycan include a jelly-roll type electrode assembly according to thepresent invention, a shape of the battery is not limited to anyparticular shape. Further, the present invention is also applicable to aprimary battery.

FIG. 1 is a cross-sectional view of a cylindrical secondary battery witha jelly-roll type electrode assembly 20 according to an aspect of theinvention. Referring to FIG. 1, the electrode assembly 20 is placed in acylindrical can 1, and the electrode assembly 20 is a rolled multi-layerstructure in which a first electrode strip 21 and a second electrodestrip 31 are positioned and a separator 40 is interposed between thefirst and second electrode strips 21 and 31 so as to insulate them fromeach other. The cylindrical can 1 is formed of a metal material so thatit acts as a connector to the first or second electrode strip 21 or 31.The can 1 is preferably formed of aluminum or an aluminum alloy andelectrically connected to the second electrode strip 31 to form a secondelectrode connector.

In the electrode assembly 20, a first electrode tab 25 and a secondelectrode tab 35 are drawn out from the first and second electrodestrips 21 and 31, respectively. More specifically, the first electrodetab 25 is drawn out from a center portion of the electrode assembly 20and the second electrode tab 35 is drawn out from an edge of theelectrode assembly 20. The first electrode tab 25 is drawn out upward tobe electrically connected to a cab assembly 2 for sealing the can 1. Thesecond electrode tab 35 is drawn out downward to be electricallyconnected to a bottom 3 of the can 1. The cab assembly 2 connected tothe first electrode tab 25 acts as a first electrode connector and thecan 1 connected to the second electrode tab 35 acts as the secondelectrode connector.

Referring to FIG. 2, an electrode assembly 20 according to an aspect ofthe invention includes a first electrode strip 21 and a second electrodestrip 31. In this embodiment, the first electrode strip 21 and thesecond electrode strip 31 act as a positive electrode strip and anegative electrode strip, respectively. However, the invention is notlimited to the above description. That is, the first electrode strip 21and the second electrode strip 31 may act as a negative electrode stripand a positive electrode strip, respectively.

As shown in FIG. 2, the first electrode strip 21 includes a firstelectrode collector 22 and an electrode mixture 23 containing anelectrode active material, and the second electrode strip 31 includes asecond electrode collector 32 and an electrode mixture 33 containing anelectrode active material. FIG. 2 illustrates a center portion of therolled electrode assembly 20 according to an aspect of the invention.FIG. 3 illustrates a method of rolling the electrode assembly 20 of FIG.2, according to an aspect of the invention.

Hereinafter, the electrode assembly 20 will be described with referenceto FIGS. 2 and 3 in a greater detail.

When the electrode assembly 20 is used in conjunction with a lithiumsecondary battery, the first electrode strip 21 may be used as apositive electrode strip. In this case, the first electrode collector 22is formed using a film type thin metal plate, preferably, a thinaluminum film. The first electrode mixture 23, which is formed at leastone surface of the first electrode collector 22, may be a mixture of afirst electrode active material (a lithium-based oxide), a binder, aplasticizer, and/or a conductive material. The first electrode collector22 has at least one surface covered with the first electrode mixture 23and an exposed portion 24, which is not coated with the first electrodemixture 23. The exposed portion is preferably a front end.

The second electrode strip 31, i.e., a negative electrode strip, mayinclude the second electrode collector 32 made of a thin metal film,preferably copper. The second electrode mixture 33, which is formed atat least one surface of the second electrode collector 32, may be madeof a mixture of a carbon-based second electrode active material, abinder, a plasticizer, and/or a conductive material. The secondelectrode collector 32 includes at least one surface covered with thesecond electrode mixture 33 and includes an exposed portion (not shown),which is not covered with the second electrode mixture 33, at theelectrode assembly 20. As shown in FIG. 1, the exposed portion 34 may bewelded together with a second electrode tab 35. Also, as shown in FIG.2, the second electrode collector 32 includes an exposed portion 34,which is not covered with the second electrode mixture 33, at its frontend, the exposed portion 34 extending beyond a center portion of theelectrode assembly 20, thus allowing the electrode assembly 20 to bemore easily inserted into and rolled by a mandrel.

A first electrode tab 25 attached to the exposed portion 24 may beformed of aluminum material and a second electrode tab 35 attached tothe outermost winding of the electrode assembly 20 may be formed of anickel material.

The electrode assembly 20 is a rolled multi-layer structure in which thefirst electrode strip 21, at least a separator 40, and the secondelectrode strip 31 are sequentially positioned. The separator 40insulates the first electrode strip 21 and the second electrode strip 31from each other and allows for an exchange of active material ionsbetween the first and second electrode strips 21 and 31, thus causing abattery reaction. The separator 40 is preferably long enough tocompletely insulate the first and second electrode strips 21 and 31 fromeach other during a contraction or expansion phase.

If the separator 40 is applicable to a lithium secondary battery, thetype of the separator 40 is not limited. For example, the separator 40may be formed of a material selected from a group includingpolyethylene, polypropylene, and a copolymer of polyethylene andpolypropylene. Otherwise, the separator 40 may be formed of apolyolefin-based polymer with a molecular weight of at least 350thousand or be a multi-layered polymer.

The separator 40 may be made by biaxially orienting a gel-type sheet andremoving a solvent from the sheet. Here, the gel-type sheet is formedusing a composition (solution) with 18 percentage by weight of UHMWPEwith an average modular weight from 200 thousand to 250 thousand and acomposition (solution) with 82 percentage by weight of HDPE with anaverage modular weight from 40 thousand to 50 thousand. Alternatively,the separator 40 may be a three-layered structure of polypropylene,polyethylene, and polypropylene, wherein a diameter of a micro hole ofeach element ranges from 0.01 μm to 0.05 μm. Otherwise, the separator 40may be made by fusing two sheets of polyethylene, or be made of a sheetof polyethylene. The fusing may be performed using heat.

The electrode assembly 20 is placed into the can 1, wherein the can 1 isfilled with an electrolyte solution. The electrolyte solution may be amixture (solution) of lithium salt and a carbonate-based organicsolvent. Further, a halogenized aromatic compound such as fluorinatedbenzene may be added to the electrolyte solution.

According to the invention, the separator 40 includes two sheets of thefirst and second separators 41 and 42. The first electrode strip 21 andthe second electrode strip 31 are formed between the first and secondseparators 41 and 42 and on sides of the first or second separator 41 or42, respectively. As shown in FIGS. 2 and 3, the first separator 41 issandwiched between the first and second electrode strips 21 and 31 andthe second separator 42 is formed at a side of the first electrode strip21. Accordingly, since the electrode assembly 20 is rolled using the twosheets of first and second separators 41 and 42, the first and secondelectrode strips 21 and 31 are insulated from each other. However,because the first and second electrode strips 21 and 31 are insulatedfrom each other, arrangement of the first and second separators 41 and42 is not limited to the above description. For instance, it is possibleto manufacture the electrode assembly 20 to include a sheet of separatorbetween the first and second electrode strips 21 and 31 or on a side ofone of the first and second electrode strips 21 and 31.

The first separator 41 includes a first inter-electrode strip separator41 a and a first protective separator 41 b which is extended from thefirst inter-electrode strip separator 41 a. The second separator 42includes a second inter-electrode strip separator 42 a and a secondprotective separator 42 b, which is extended from the secondinter-electrode strip separator 42 a. The first and secondinter-electrode strip separators 41 a and 42 a are used as separationfilms of the secondary battery, allowing for an exchange of activematerial ions between the first and second electrode strips 21 and 31coated with the first and second electrode mixtures 23 and 33,respectively. As shown in FIG. 2, the first and second protectiveseparators 41 b and 42 b are extended from the first and secondinter-electrode strip separators 41 a and 42 a, respectively. At leastone of the first and second protective separators 41 b and 42 b isfurther positioned on a side of the first electrode strip 21 attachedwith the first electrode tab 25, thereby preventing electricalshort-circuiting between the first and second electrode strips 21 and31.

It is preferable that the first and second protective separators 41 band 42 b are ends of the first and second inter-electrode stripseparators 41 a and 42 a and further positioned on the opposite side ofthe side of the first electrode strip 21 attached with the firstelectrode tab 25. Referring to FIG. 2, the second electrode strip 31 maybe positioned on the opposite side of the side of the first electrodestrip 21 attached with the first electrode tab 25. In this way, it ispossible to prevent electrical short-circuiting between the first andsecond electrode strips 21 and 31 resulting from damage to the separatorcaused by heating of the first electrode tab 25 attached to the firstelectrode strip 21 due to a concentration of electric charges.

In other words, as shown in FIG. 2, the first and second protectiveseparators 41 b and 42 b, which are the ends of the first and secondinter-electrode strip separators 41 a and 42 a, are further interposedbetween the side of the first electrode strip 21 attached with the firstelectrode tab 25 and the second electrode strip 31, thus increasinginsulation between the first and second electrode strips 21 and 31.Thus, even if the inter-electrode strip separators 41 a and 42 a aredamaged due to heat, the electrical short-circuiting may be avoidedbetween the first and second electrode strips 21 and 31 because of thefirst and second protective separators 41 b and 42 b.

In addition, it is possible to prevent electrical short-circuitingbetween the first and second electrode strips 21 and 31 caused by abreaking of the separator due to a surface defect, such as a burr, ofthe first electrode tab 25. As shown in FIG. 2, only the separators arelocated on the opposite side of the side of the first electrode strip 21attached with the first electrode tab 25 so as to completely preventelectrical short-circuiting at the side of the first electrode strip 21with the first electrode tab 25.

In this embodiment, as shown in FIG. 2, both or one of the first andsecond protective separators 41 b and 42 b may be positioned on the sideof the first electrode strip 21 with the first electrode tab 25.

As shown in FIG. 2, the first and second protective separators 41 b and42 b are bent twice while being extended from the first and secondinter-electrode separators 41 a and 42 a, respectively, extended whilebeing wound in the opposite direction of rolling the electrode assembly20, and then are positioned on the opposite side of a side of the firstinter-electrode strip separator 41 a, toward the center portion of theelectrode assembly, between the first and second electrode strips 21 and31. Although not shown in the drawings, when only the second protectiveseparator 42 b is extended and positioned on the opposite side of thefirst inter-electrode strip separator 41 a between the first and secondelectrode strips 21 and 31, the second protective separator 42 b may beextended to the first electrode tab 25 while being inserted between thefirst inter-electrode strip separator 41 a and the first electrode strip21.

Also, although not shown in the drawings, both the first and secondinter-electrode strip separators 41 a and 42 a may be positioned on theopposite side of the side of the first electrode collector 22 attachedwith the first electrode tab 25. Also, a portion of the first electrodecollector 22 may be exposed while at least two separators, e.g., thefirst and second inter-electrode strip separators 41 a and 42 a, arepositioned on the opposite side of the side of the first electrode strip21 attached with the first electrode tab 25.

General equipment used for rolling the electrode assembly 20 may be usedfor the insertion of the first and second protective separators 41 b and42 b. For instance, when rolling the electrode assembly 20, using adevice such as the mandrel 50 shown in FIG. 3, the electrode assembly 20is fabricated by inserting two sheets of the first and second separators41 and 42 into the mandrel 50, sandwiching the first electrode strip 21between the first and second separators 41 and 42, placing the secondelectrode strip 31 on the other side of the first separator 41, androtating the mandrel 50 in a direction as shown by the arrow in FIG. 3.

More specifically, the first and second separators 41 and 42 areinserted into the mandrel 50, a portion of each of these separators 41and 42 is extended out from an end of the mandrel 50 by a length L1, andthe mandrel 50 is rotated. Then, the portions of the lengths L1 of thefirst and second separators 41 and 42, which pass through the mandrel50, are rotated in the opposite direction in which the mandrel 50 isrolled, as shown in FIG. 3. The rotated portions of the first and secondseparators 41 and 42 are extended to the portion of the first electrodestrip 21 attached with the first electrode tab 25, so that an upperportion of the first separator 41 is covered with these portions. Next,the mandrel 50 is kept rolled to obtain the first and second protectiveseparators 41 b and 42 b of FIG. 3.

The lengths L1 of the portions of the first and second separators 41 and42, which are drawn out from the mandrel 50, must be at least half alength of the mandrel 50 in circumference. In this case, as shown inFIG. 3, the first electrode tab 25 is welded at a front end of theexposed portion 24 and the electrode assembly 20 is rolled while a frontend of the first electrode tab 21 is placed adjacent to a mouth of themandrel 50. More specifically, the lengths L1 of the portions of thefirst and second separators 41 and 42 must be longer than a sum of halfthe length of the mandrel 50 in circumference and a distance between themandrel 50 and the side of the first separator 41 attached with thefirst electrode tab 25, so that the upper portion of the first separator41 attached with the first electrode tab 25 is entirely covered with andprotected by the portions of the first and second separators 41 and 42.

If the lengths L1 of the portions of the first and second separators 41and 42 are extremely long, it unnecessarily increases the thickness ofthe electrode assembly 20. As shown in FIG. 3, the lengths L1 arepreferably determined such that the first and second protectiveseparators 41 b and 42 b, of the first and second separators 41 and 42,which are drawn out from the mandrel 50, are one-half a circumference ofthe mandrel 50 and are extended to a beginning of the second electrodestrip 31. In other words, the lengths L1 are preferably equal to or lessthan a sum of half the circumference of the mandrel 50 and the distancebetween the mandrel 50 and an end of the second electrode strip 31nearest to the mandrel 50.

After rolling the electrode assembly 20, the mandrel 50 is removed toobtain the electrode assembly 20 shown in FIG. 2. Otherwise, the mandrel50 may be kept to be inserted into the electrode assembly 20.

Meanwhile, as shown in FIG. 2, it is possible to further preventelectrical short-circuiting between the first and second electrodestrips 21 and 31 by welding the first electrode tab 25 to a side of thefirst electrode collector 22 toward the center of the electrode assembly20.

In detail, when the first electrode tab 25 is welded to the side of thefirst electrode collector 22 toward the center of the electrode assembly20, the first electrode tab 25 confronts the center of the electrodeassembly 20 and only the first or second separator 41 or 42 contacts thefirst electrode tab 25. Therefore, even if the first electrode tab 25includes a surface defect such as a burr, electrical short-circuiting isnot caused.

Further, even if the burr of the first electrode tab 25 projects outsidethe electrode assembly 20, electrical short-circuiting does not occurunless all the first electrode collector 22 and the first and secondprotective separators 41 b and 42 b are damaged.

In addition, installation of the first and second protective separators41 b and 42 b at the opposite side of the side of the first electrodestrip 21 attached with the first electrode tab 25 prevents electricalshort-circuiting between the first and second electrode strips 21 and 31caused by heating of the first electrode tab 25.

As described above with reference to FIG. 2, the invention is describedwith respect to a case where the first electrode tab 25 is welded to theside of the first electrode collector 22 facing the center portion ofthe electrode assembly 20. Alternatively, as shown in FIG. 4, electricalshort-circuiting may be prevented by welding the first electrode tab 25to a side of the first electrode collector 22 facing an outer side ofthe electrode assembly 20.

When protecting the side of the first electrode strip 21 attached withthe first electrode tab 25 using the first and second protectiveseparators 41 b and 42 b, starting points of the electrode mixtures 23and 33 of the electrode strips 21 and 31 are constructed as shown inFIGS. 2 through 4. In this case, a battery reaction area is increased byrolling the electrode assembly 20, even to a small degree, therebymaximizing battery capacity. Although not shown in the drawings, an areaavailable for battery reaction may be increased by forming a startingpoint of the first electrode strip 21 where the first electrode mixture23 starts to be adjacent to the first electrode tab 25.

An active material may not be applied on a side of the first or secondelectrode strip 21 or 31 that does not contact an active material of thefirst or second electrode strip 21 or 31 of the opposite polarity viathe first or second separator 41 or 42. More specifically, as shown inFIG. 5, an active material layer 33 a and another active material layer33 b are positioned on a side of a second electrode collector 32 facinga center portion of the electrode assembly 20 and the opposite sidethereof, respectively. Starting points of the active material layers 33a and 33 b, at a front end of the second electrode strip 31, that isprovided on the first electrode strip 21 via the separators 41 and 42toward the center of the rolled electrode assembly 20, are differentfrom each other. A side of the front end of the second electrode strip31, which is unavailable for battery reaction, is not covered with anelectrode mixture. Accordingly, increasing the volume of the electrodeassembly 20 is not necessary. Such asymmetrical application of an activematerial is applicable to not only the first and second electrode strips21 and 31 but also an outermost surface of a battery.

According to the invention, starting points of the first and secondelectrode strips 21 and 31, which are covered with an electrode mixtureor not covered with the electrode mixture, may be set in variouspositions or locations.

FIG. 6 is a cross-sectional view of a center portion of an electrodeassembly 20 according to an aspect of the invention. Referring to FIG.6, in addition to the first and second protective separators 41 b and 42b, an exposed portion 24 of the first electrode strip 21 of a firstelectrode collector 22 may be positioned on the side of a firstelectrode strip 21 attached with a first electrode tab 25 in order toprevent electrical short-circuiting at the side of the first electrodestrip 21 attached with the first electrode tab 25.

More specifically, a portion 24 a extended from the exposed portion 24is positioned between the first and second protective separators 41 band 42 b on the other side of a side of a first inter-electrode stripseparator 41 a facing the opposite side of the side of the firstelectrode strip 21 attached with the first electrode tab 25.Accordingly, even if first and second separators 41 and 42 located onthe side of the electrode strip 21 attached with the first electrode tab25 are removed or torn-out, the portion 24 a of the same polarity ispositioned on the other side of the first electrode strip 21, thuspreventing of electrical short-circuiting between the first and secondelectrode strips 21 and 31.

FIG. 7 illustrates a method of rolling of the electrode assembly of FIG.6 according to an aspect of the invention. As described above withreference to FIG. 3, the first and second protective separators 41 b and42 b are inserted into a mandrel 50 and their respective portions aredrawn out from the mandrel 50 by a length L1. Next, a portion 24 a ofthe exposed portion 24 of the first electrode strip 21 between the firstand second separators 41 and 42, is inserted into the mandrel 50 and theportion 24 a is drawn out from the mandrel 50 by a length L2. The drawnportions of the first and second protective separators 41 b and 42 b andthe drawn portion of the portion 24 a are then rolled by rotating themandrel 50 in a predetermined direction. For example, the mandrel 50 maybe rolled in a direction as shown by the arrow as shown in FIG. 7. Thelength L2 of the portion 24 a passing through the mandrel 50 must be atleast half the circumference of the mandrel 50. More particularly, thelength L2 must be longer than a sum of half the length of the mandrel 50in circumference and a distance between the mandrel 50 and the firstelectrode tab 25. Also, the length L2 of the portion 24 a must beshorter than the lengths L1 of the first and second protectiveseparators 41 b and 42 b passing through the mandrel 50. If the lengthL2 is longer than the length L1, the portion 24 a may contact the secondelectrode strip 31 when the portion 24 a is rolled back by the mandrel50.

Similar to the rolled portions of the first and second protectiveseparators 41 b and 42 b, the rolled portion of the portion 24 a is benttwice by the mandrel 50 and positioned on the opposite side of the sideof the first electrode strip 21 attached with the first electrode tab25. That is, the portion 24 a is extended to the side of the firstelectrode strip 21 attached with the first electrode tab 25 while beingwound in the opposite direction in which the electrode assembly 20 isrolled.

The method of FIG. 7 includes a situation where the first electrode tab25 is welded to a side of the first electrode collector 22 toward acenter portion of the first electrode assembly 20. However, the methodof FIG. 7 is also applicable to a situation where the first electrodetab 25 may be welded to the other side of the first electrode collector22, as shown in FIG. 8.

Alternatively, as shown in FIG. 9, only the first protective separator41 a may be extended to the side of the first electrode strip 21attached with the first electrode tab 25 while the portion 24 a is notpositioned between the first and second protective separators 41 b and42 b. In this case, the portion 24 a of the exposed portion 24 protectsthe opposite side of the side of the first electrode strip 21 attachedwith the first electrode tab 25.

As describe above, in the electrode assembly 20, an electrode mixturemay not be applied onto a side of the first or second electrode strip 21or 31 that does not contact an active material layer of the otheradjacent electrode strip 21 or 31 of different polarity and theseparators 41 and 42. That is, referring to FIG. 10, the active materiallayer 33 a is positioned on a side of the second electrode collector 32facing the center of the electrode assembly 20 and the active materiallayer 33 b is positioned on the other side. Starting points of theactive material layers 33 a and 33 b are different from each other at anend of the second electrode strip 31, which resides on the firstelectrode strip 21 via the separators 41 and 42. An electrode mixture isnot applied onto a front side of the first or second electrode strip 21or 31 that is not available for battery reaction. Accordingly, volume ofthe electrode assembly 20 is not unnecessarily increased. Suchasymmetrical application of the active material is also applicable tothe outermost surface of a battery.

The invention is applicable to manufacturing various shapes, sizes, andtypes of electrode assembly, including the cylindrical batteries shownin FIGS. 1 through 10. For example, a rectangular or pouch-type batterymay be fabricated by applying pressure onto an electrode assemblyaccording to the present invention and placing the flattened electrodeassembly into a rectangular can or a pouch case.

In addition to the method described above of protecting the side of thefirst electrode strip 21 attached with the first electrode tab 25, it isalso possible to protect a side of the second electrode strip 31attached with the second electrode tab 35.

Table 1 shows results obtained when performing several tests to evaluatethe effectiveness of a secondary battery with a structure shown in FIG.2 compared to that of a conventional secondary battery in which a sheetof separator is positioned on a side of an electrode strip attached withan electrode tab. Five secondary batteries according to the presentinvention and five conventional secondary batteries were used in thetests.

Secondary Batteries Conventional According to Secondary the InventionBatteries Item L0 L1 L2 L3 L4 L5 L0 L1 L2 L3 L4 L5 Overcharge Test 5 5Penetration Test 5 5 Crush Test 5 2 3 Impact Test 5 3 2 Drop Test (1.9M) 5 5 Drop Test (10 M) 5 5 Thermal 13 10 Exposure Test min. min. or ormore more

In Table 1, the secondary batteries according to the present inventionand the conventional secondary batteries include the same type ofseparators. In particular, a sheet of separator is installed at a sideof an electrode strip attached with an electrode tab of each of theconventional secondary batteries, the side facing a center of a rolledelectrode assembly. Three sheets of separators are installed at a sideof an electrode strip attached with an electrode tab of each of thesecondary batteries according to the present invention, the side facinga center of an electrode assembly.

In the overcharge test of Table 1, the secondary batteries and theconventional batteries were charged to 250 percent of the rated capacityand exteriors of the batteries were observed.

In the penetration test, after standard charging of the secondarybatteries and the conventional secondary batteries, the charging stoppedwithin a range from 10 minutes to 72 hours. Next, the batteries wereplaced on a plane surface, a conductive tool, such as a nail or achisel, was penetrated into the batteries, and the penetration wasmaintained until their surface temperatures reached 40° C. or less. Theconductive tool was then removed from the batteries and the batterysurface temperatures and exteriors were observed for 10 minutes.

In the impact test, after standard charging of the secondary batteriesand the conventional secondary batteries, the charging stopped within arange from 10 minutes to 72 hours. Next, each battery was placed on aplane surface and fixed with a jig, a bar with a diameter of 15.8 mm waslaid on the battery approximately perpendicular to the battery in alengthwise direction., An object of 9.1 kg was then dropped on thebattery from a height of 61 cm. Thereafter, the battery was left aloneuntil the battery surface temperature reached 40° C. or less, the jigwas removed, and the battery surface temperature and exterior wereobserved for 10 minutes.

In the crush test, after standard charging of the secondary batteriesand the conventional secondary batteries, the charging stopped within arange from 10 minutes to 72 hours. Each battery was placed between twosheets of steel plates while being fixed with a jig, and a force of 13KN was applied onto the battery or a pressure was applied onto thebattery to press it to two-thirds of the original thickness. Thereafter,the battery was left alone until its surface temperature reaches 40° C.or less, the jig was removed, and then, the battery surface temperatureand exterior were observed for 10 minutes.

In the drop test, the secondary batteries and the conventional batterieswere dropped from heights of 1.9 m and 10 m, and their exteriors wereobserved.

In the thermal exposure test, after standard charging of the secondarybatteries and the conventional ones, the charging was stopped within arange from 10 minutes to 72 hours. Each battery was then placed in achamber, battery surface temperature was increased by 5° C. per minute,and the battery surface temperature and exterior were observed whilemaintaining current battery state for 1 hour when the battery surfacetemperature reached 150° C.

In Table 1, L0 includes a situation where there is no change regardingthe exteriors of the batteries. In addition, L0 includes cases wherethere is no flash of light even when battery leakage is detected, andthe battery exteriors are changed due to impacts applied during thetests while the batteries are remain hermetic.

In Table 1, L1 includes situations where the original battery weightdecreases by at least 0.1 percent due to loss of an inner part ofbattery such as an electrolyte, a ruptured vent, and/or electrolyteleakage is visually detected. In general, electrolyte leakage isdetermined by measuring the weight of a battery after a test andcomparing the measured weight with the original weight. However, such aprocess may be omitted from a battery safety test. Further, L1 includesleakage of electrolyte from the inside of battery due to batterycomponents breakage caused by impacts applied during a test withoutcausing smoke, emission of gas, or fire.

L2 includes a situation where smoke, such as vapor, emits from theinside of a battery without causing sudden heating and battery surfacetemperature is less than 200° C.

L3 includes a situation where smoke emits from the inside of a batterycausing sudden heating, battery surface temperature is more than 200°C., gas emits intensely, and a battery surface temperature is greaterthan 200° C. due to sudden heating of the battery, even if no smoke isgenerated from the side of battery.

L4 includes a situation where battery contents are spontaneously ignitedand burned causing violent flames or of the exterior of the battery isburned.

L5 includes a situation where battery contents come out from a batterydue to a pressure applied from the inside of the battery, and portionsof a battery case are broken into pieces, the portions do not include avent of a battery.

In the thermal exposure test, a time required for each battery to enterthe situation L5 is measured, starting from when the temperature of achamber reaches 150° C.

As is apparent from Table 1, the secondary batteries and theconventional batteries showed the same results in the overcharge test,the penetration test, and the drop test. However, in the crush test,battery leakage was detected from all five secondary batteries, butbattery leakage was detected from two of the conventional batteries andflashes of light were generated in the other three conventionalbatteries. In the impact test, none of the five secondary batteriesshowed any sign of burning but two of the conventional batteries ignitedcausing flames. In the thermal exposure test, the five secondarybatteries exploded at least 13 minutes after the temperature of achamber reached 150° C., but the conventional batteries exploded only 10minutes after the temperature of a chamber reached 150° C.

The test results shown in Table 1 revealed that including an electrodeassembly, according to the invention, into a secondary battery improvesbattery safety. In particular, the thermal exposure test showed that theexplosion of a secondary battery including the electrode assemblyaccording to the invention may be delayed by at least 3 minutes comparedto that of a conventional battery.

As described above, a secondary battery according to the invention hasthe following advantages. First, the secondary battery does not under gomuch electrical short-circuiting between electrodes as a result of aseparator breaking caused by heating a side of an electrode stripattached with an electrode tab. Second, electrical short-circuitingbetween electrodes may be prevented even if an electrode tab includes asurface defect, such as a burr. Third, battery safety is improved. Four,electrical short-circuiting may be prevented at a side of an electrodestrip attached with an electrode tab even if a secondary battery ismanufactured using conventional equipment. Fifth, the volume of anelectrode assembly is not unnecessarily increased by having startingpoints of electrode mixtures positioned on first and second electrodestrips the same.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An electrode assembly comprising: a first electrode strip comprisinga first electrode collector coated with a first electrode activematerial; a first electrode tab attached to an exposed portion of thefirst electrode collector; a second electrode strip comprising a secondelectrode collector coated with a second electrode active material; asecond electrode tab attached to an exposed portion of the secondelectrode collector; and an inter-electrode strip separator disposedbetween the first and second electrode strips, having an end that isbent to form a protective separator, wherein, the first electrode strip,the second electrode strip, and the inter-electrode separator are rolledtogether in a first direction, and the protective separator is rolled inan opposing second direction.
 2. The electrode assembly of claim 1,wherein the first electrode tab is located adjacent to a center of therolled electrode assembly.
 3. The electrode assembly of claim 2, whereinthe inter-electrode strip separator and the protective separator arepositioned on opposing sides of the first electrode collector.
 4. Theelectrode assembly of claim 1, wherein the protective separator isinserted between the first electrode tab and the second electrode strip.5. The electrode assembly of claim 4, wherein the protective separatoris bent, so as to extend across a center of the electrode assembly. 6.The electrode assembly of claim 1, wherein the first electrode tab isattached to a side of the first electrode collector that faces a centerof the electrode assembly.
 7. The electrode assembly of claim 1, whereinthe protective separator and the inter-electrode strip separator areoverlapped and inserted between the first electrode tab and the secondelectrode strip.
 8. The electrode assembly of claim 1, furthercomprising another inter-electrode strip separator having an end that isbent to form another protective separator, wherein the inter-electrodestrip separator, the first electrode strip, the other inter-electrodestrip separator, and the second electrode strip are sequentially stackedand rolled in the first direction, and the other protective separator isrolled in the second direction.
 9. The electrode assembly of claim 1,wherein a starting position of each of the first and second activematerials is the same.
 10. The electrode assembly of claim 1, whereinthe first electrode tab is attached to an inner side of the firstelectrode collector, which faces a center of the electrode assembly. 11.The electrode assembly of claim 10, wherein the protective separatorextends along an opposing outer side of the first electrode strip. 12.The electrode assembly of claim 1, wherein the inter-electrode stripseparator comprises at least one material selected from a groupincluding polyethylene, polypropylene, and a copolymer of polyethyleneand polypropylene.
 13. The electrode assembly of claim 1, wherein atleast one of the inter-electrode strip separator and the protectiveseparator has a multi-layered structure.
 14. The electrode assembly ofclaim 1, wherein the inter-electrode strip separator comprises apolyolefin-based polymer having a molecular weight of at least 350thousand.
 15. The electrode assembly of claim 1, wherein the exposedportion of the first electrode collector and the protective separatorare both bent and rolled in the second direction, and inserted betweenthe first electrode tab and the second electrode strip.
 16. Theelectrode assembly of claim 1, wherein the first electrode activematerial comprises a lithium-based oxide, and the second electrodeactive material comprises a carbon-based material.
 17. The electrodeassembly of claim 1, wherein the first electrode collector is formed ofaluminum or an aluminum alloy, and the second electrode collector isformed of copper or a copper alloy.
 18. A secondary battery includingthe electrode assembly of claim 1, further comprising a cylindrical, arectangular, or a pouch-type case, to house the electrode assembly. 19.An electrode assembly comprising: a first electrode strip comprising afirst electrode collector coated with a first electrode active material;a first electrode tab attached to an exposed portion of the firstelectrode collector; a second electrode strip comprising a secondelectrode collector coated with a second electrode active material; asecond electrode tab attached to an exposed portion of the secondelectrode collector; and an inter-electrode strip separator disposedbetween the first and second electrode strips, having an end that isbent to form a protective separator, wherein, the first electrode strip,the second electrode strip, and the inter-electrode separator are rolledtogether in a first direction, and the protective separator and theexposed portion of the first electrode collector are bent and rolled inan opposing second direction.
 20. The electrode assembly of claim 19,wherein the first electrode tab is positioned adjacent to a center ofthe electrode assembly.
 21. The electrode assembly of claim 20, whereinthe inter-electrode strip separator and the protective separator areoverlapped and inserted between the second electrode strip and the firstelectrode tab.
 22. The electrode assembly of claim 19, wherein theprotective separator extends across a center of the electrode assembly.23. The electrode assembly of claim 22, wherein the protective separatoris bent more than one time and inserted between the first electrode taband the second electrode strip.
 24. The electrode assembly of claim 19,wherein the exposed portion of the first electrode collector is insertedbetween first electrode tab and the second electrode strip.
 25. Theelectrode assembly of claim 24, wherein the exposed portion of the firstelectrode collector is bent more than one time and inserted between thefirst and second electrode strips.
 26. The electrode assembly of claim19, wherein the first electrode tab is attached to an inner side of thefirst electrode collector, which faces a center of the electrodeassembly.
 27. The electrode assembly of claim 19, wherein the protectiveseparator is inserted between the first electrode tab and the secondelectrode strip.
 28. The electrode assembly of claim 19, wherein thefirst electrode collector is attached to a side of the first electrodetab, which faces a center of the electrode assembly.
 29. The electrodeassembly of claim 19, further comprising another inter-electrode stripseparator disposed between the first and second electrode strips, havingan end that is bent to form another protective separator that isinserted between the first electrode tab and the second electrode strip,wherein the inter-electrode strip separator, the first electrode strip,the other inter-electrode strip separator, and the second electrodestrip are sequentially stacked and rolled in the first direction, andthe other protective separator is rolled in the second direction. 30.The electrode assembly of claim 19, wherein a starting position of eachof the first and second electrode active materials is the same.
 31. Theelectrode assembly of claim 19, wherein the inter-electrode stripseparator and the protective separator comprise at least one materialselected from a group including polyethylene, polypropylene, and acopolymer of polyethylene and polypropylene.
 32. The electrode assemblyof claim 19, wherein the inter-electrode strip separator and theprotective separator are overlapped and inserted between the firstelectrode tab and the second electrode strip.
 33. The electrode assemblyof claim 19, wherein at least one of the inter-electrode strip separatorand the protective separator comprises a polyolefin-based polymer havinga molecular weight of at least approximately 350 thousand.
 34. Theelectrode assembly of claim 19, wherein the first electrode activematerial comprises a lithium-based oxide, and the second electrodeactive material comprises a carbon-based material.
 35. The electrodeassembly of claim 19, wherein the first electrode collector is formed ofaluminum or an aluminum alloy, and the second electrode collector isformed of copper or a copper alloy.
 36. A secondary battery includingthe electrode assembly of claim 19, further comprising a cylindricalcase, a rectangular case, or a pouch-type case, to house the electrodeassembly.
 37. An electrode assembly comprising: a first electrode stripcomprising a first electrode collector coated with a first electrodeactive material; a first electrode tap attached to an uncoated exposedportion of the first electrode collector; a second electrode stripcomprising a second electrode collector coated with a second electrodeactive material; a second electrode tab attached an uncoated exposedportion of the second electrode collector; and an inter-electrode stripseparator disposed between the first and second electrode strips,wherein, the first electrode strip, the second electrode strip, and theinter-electrode separator are rolled together in a first direction, andthe exposed portion of the first electrode collector extends across acenter cavity of the electrode assembly and is then rolled in anopposing second direction and inserted between the second electrodestrip and the first electrode tab.
 38. An electrode assembly comprising:a first electrode strip comprising a first electrode collector coatedwith a first electrode active material; a first electrode tab attachedto an exposed portion of the first electrode collector; a secondelectrode strip comprising a second electrode collector coated with asecond electrode active material; and an inter-electrode strip separatordisposed between the first and second electrode strips, theinter-electrode strip separator having an end portion that is bent toform a protective separator, wherein the protective separator isinserted between the first electrode tab and the second electrode strip.